Half-Life 2: Episode One (originally known as Half-Life 2: Aftermath) is the first installment of the revolution series, or episodes, the second installation of the official Half-Life 2 series, and the fifth installment in the Half-Life series. It was released on PlayStation 3 and Xbox 360 as part of The Orange Box.

Plot summary

This section contains spoilers! Content within this section may reveal significant parts of a game(s) story.

Gordon Freeman and Alyx Vance must escape City 17 before the Combine Citadel's reactor goes critical and destroys the city, but first, they must stabilize the reactor to buy sufficient time to escape. Along the way, they assist Barney Calhoun with his efforts to rescue some of City 17's other inhabitants. Eventually, Gordon and Alyx make it out of City 17, but the Citadel exploded behind them; the game then ends with a black screen.